netdev - Re: [RFC v2] fq_codel : interval servo on hosts

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Date:	Tue, 4 Sep 2012 08:10:36 -0700
From:	Nandita Dukkipati <nanditad@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Yuchung Cheng <ycheng@...gle.com>, codel@...ts.bufferbloat.net,
	Dave Taht <dave.taht@...il.com>,
	Kathleen Nichols <nichols@...lere.com>,
	netdev <netdev@...r.kernel.org>, Tomas Hruby <thruby@...gle.com>
Subject: Re: [RFC v2] fq_codel : interval servo on hosts

The idea of using srtt as interval makes sense to me if alongside we
also hash flows with similar RTTs into same bucket. But with just the
change in interval, I am not sure how codel is expected to behave.

My understanding is: the interval (usually set to worst case expected
RTT) is used to measure the standing queue or the "bad" queue. Suppose
1ms and 100ms RTT flows get hashed to same bucket, then the interval
with this patch will flip flop between 1ms and 100ms. How is this
expected to measure a standing queue? In fact I think the 1ms flow may
land up measuring the burstiness or the "good" queue created by the
long RTT flows, and this isn't desirable.

On Sat, Sep 1, 2012 at 5:51 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Fri, 2012-08-31 at 18:37 -0700, Yuchung Cheng wrote:
>
>> Just curious: tp->srtt is a very rough estimator, e.g., Delayed-ACks
>> can easily add 40 - 200ms fuzziness. Will this affect short flows?
>
> Good point
>
> Delayed acks shouldnt matter, because they happen when flow had been
> idle for a while.
>
> I guess we should clamp the srtt to the default interval
>
> if (srtt)
>         q->cparams.interval = min(tcp_srtt_to_codel(srtt),
>                                   q->default_interval);
>
>
>
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